Plumbing associated infections cause tens of thousands of illnesses and deaths every year. Clinically significant plumbing-associated pathogens include Gram-negative environmental bacteria and free-living amoeba (FLA) that can enter plumbing systems in relatively small numbers, reproduce (amplify) to large numbers and release as respirable bio-aerosols from the plumbing into the environment. The only plumbing-associated disease requiring notification in the United States is Legionnaires' disease, a severe pneumonic infection caused by the bacterium Legionella. Premise plumbing systems are now recognized as the primary source of Legionnaires' disease. (Yoder et al., 2008) The US Centers for Disease Control and Prevention (CDC) has estimated there are as many as 18,000 cases of Legionnaire's disease annually. The US Occupational Safety and Health Administration (OSHA) has estimated that Legionnaires' disease results in about 4,000 deaths in the United States each year. Reported outbreaks of Legionnaires' disease have more than doubled in the past 10 years. Other plumbing-associated pathogens, such as Pseudomonas and non-tuberculous mycobacteria (NTM), may cause as much or more disease as Legionella, but lack of required reporting and other factors make quantification difficult. The primary disease transmission vectors for these plumbing associated pathogens are inhalation and aspiration.
Since the early 20th Century, water treatment and disinfection practices implemented in the United States and other developed countries have virtually eliminated incidence of waterborne enteric diseases, such as typhoid and cholera that result from fecal contamination of the public water supply. The focus of these historic, successful efforts has been the control of “traditional pathogens”, waterborne pathogens of fecal origin that contaminate the source water and typically do not amplify in the potable water itself. The primary disease transmission vector for these traditional pathogens is ingestion.
E. coli is a reference organism of choice in traditional water treatment; it is widely used as the primary indicator of fecal contamination. Current data suggest that E. coli is almost exclusively derived from the feces of warm-blooded animals; its presence in drinking water is considered an indication of substantial post-treatment fecal contamination or inadequate treatment. E. coli is extremely sensitive to chemical disinfection, such as chlorination. Its presence in a water sample is considered a sure sign of a major deficiency in the treatment program or in the integrity of the distribution system. However, the absence of E. coli does not, by itself, provide sufficient assurance that the water is free of microbial contamination.
Constituents of water, pipe deposits and plumbing materials exert an initial chemical demand on oxidizing disinfectants, such as chlorine. The amount of disinfectant that remains after the initial oxidant demand is satisfied is called the “disinfectant residual”. “Ct”—the concentration of the disinfectant residual [C] multiplied by the contact time, “t”—is a key concept used in development of traditional disinfection protocols. Ct tables have been developed for each drinking water disinfectant for a number of challenge organisms, primarily suspended (planktonic), traditional (enteric) indicator pathogens such as E. coli and Giardia. 
In general, public drinking water supplies in developed countries are treated to government standards that make the water safe for intended use. In the United States, potable water supplied by community water systems is treated to National Primary Drinking Water Standards, a set of requirements developed by the United States Environmental Protection Agency (USEPA) under authority of the Safe Drinking Water Act (SDWA). Most regulatory mandates regarding drinking water have focused primarily on the quality of the water at the point it leaves the treatment plant.
It is increasingly recognized that the quality of regulation-compliant drinking water can deteriorate after it enters the distribution system, the series of pipes that transport water from the treatment plant to the customer. In 2006, at the request of USEPA, the National Academy of Sciences published a study by the Water Science Technology Board (WSTB) of the National Research Council (NRC), “Drinking Water Distribution Systems: Assessing and Reducing Risks”. (NRC, 2006) The study highlighted the urgent need for new science that will enable cost-effective treatment of the distribution system for protection of public health and minimization of water quality degradation after water leaves the treatment plant. The distribution system is often categorized from largest to smallest components: transmission (trunk) mains, distribution mains, service lines, and premise plumbing. Typically, the water treatment utility owns and is responsible for the distribution system infrastructure up to the connection to the customer, which sometimes includes the service line. Almost always, the customer is responsible for the premise plumbing. The study highlights treatment challenges that are unique to premise plumbing.
The term “premise plumbing” refers to the piping within a building or home that distributes water to the point of use; it also includes equipment used to process the water—that is, to soften, filter, store, heat, and circulate the water before it exits the tap. Premise plumbing systems are comprised of a wide range of materials including copper, plastics, brass, lead, galvanized iron, and occasionally stainless steel. Many of these materials typically are not present in the main distribution system. Compared to other parts of the water distribution system, premise plumbing is characterized by longer water-residence times, more stagnation, lower flow conditions, higher surface area to volume ratio (owing to relatively lengthy sections of small-diameter pipe), lower (if any) disinfectant residual and higher water temperatures. The distinctive characteristics of premise plumbing create a unique ecological niche and home to a robust microbial ecology.
The microbial colonization of plumbing systems occurs primarily through the formation of natural biofilms upon the interior surfaces of the plumbing. (Declerck, 2010; Murga et al., 2001) Biofilms are complex heterogeneous aggregates of microorganisms and exogenous materials embedded in a highly hydrated matrix commonly referred to as extracellular polymeric substances (EPS). EPS is made up of a variety of constituents, including polysaccharides, protein, lipids and nucleic acids. The development, chemical composition, microbial diversity, morphology and activity of biofilms are affected by a number of factors, including water temperature, pH, hardness, disinfection history and the composition of the plumbing surface upon which the biofilm forms. For example, biofilms that form on copper pipe in a domestic hot water system are different from the biofilms that form on the interior surfaces of transmission mains, even in the same overall water system.
Biofilm formation on a plumbing surface can be initiated when relatively small numbers of environmental microorganisms (such as are typically found in high-quality, regulation-compliant drinking water) enter the plumbing system, attach to the inside surfaces of pipes and equipment, excrete EPS and amplify to very large numbers. Pieces of the biofilm can shed or be dislodged and broadcast as respirable droplets in infectious bio-aerosols from the plumbing into the environment, for example through showerheads, faucet fixtures and ornamental fountains. Infection by these bio-aerosols is primarily by inhalation and aspiration, and sometimes wound infection.
Clinically important biofilm-associated microorganisms that colonize the interior surfaces of premise plumbing systems include Gram-negative environmental bacteria, such as Legionella, Acinetobacter, Elizabethkingia (Flavobacterium), Stenotrophomonas, Klebsiella, Pseudomonas and NTM.
Legionella, the most studied plumbing-associated pathogen, survives over a wide range of temperatures. It is acid tolerant to pH 2.0 (Anand et al., 1983) and able to survive temperatures of up to 70° C. (158° F.) (Sheehan et al., 2005). Subject to the availability of necessary nutrients (e.g., iron, L-cysteine), Legionella can grow in water at 20-50° C. Legionella proliferate vigorously in water at 32-42° C. (89.6-107.6° F.) with low levels of available nutrients, e.g., in unsterilized tap water (Yee and Wadowsky, 1982), especially in slow-flowing or stagnant water. Legionella is comparatively less susceptible to chlorination than E. coli, and reportedly can survive chlorine doses of up to 50 mg/L when contained inside protozoan hosts.
Bacteria and other biofilm-resident microorganisms often are physiologically different from their free-floating (planktonic) counterparts, and have been shown to be far more resistant to traditional disinfectants, such as chlorine. For example, biofilm bacteria grown on the surfaces of granular activated carbon particles, metal coupons, or glass microscope slides were 150 to more than 3,000 times more resistant to hypochlorous acid (free chlorine, pH 7.0) than were unattached cells. In contrast, resistance of biofilm bacteria to monochloramine disinfection ranged from 2- to 100-fold more than that of unattached cells. (LeChevallier, et al. 1988)
Protozoa play a defining role in the microbial ecology of plumbing system associated biofilms. Protozoa graze on biofilm organisms. A number of biofilm-associated pathogens (e.g., Legionella, NTM, Pseudomonas) are able to parasitize and replicate within species of FLA commonly found in drinking water. Once consumed and phagocytized by the protozoan host, these bacterial pathogens survive, replicate and are eventually dispersed to infect new hosts. While inside the host, the bacteria are protected from environmental stressors, such as disinfectants and high temperatures. In addition to promoting the bacteria's survival, this process reportedly can result in the up-regulation of the bacteria's virulence genes, and thus directly affect their ability to infect humans and cause disease. L. pneumophila has been shown able to parasitize and multiply in more than twenty different protozoan species, including Acanthamoeba, Naegleria, and Hartmanella (Donlan et al., 2005; Kuiper et al., 2004). Protozoa have been shown to be highly resistant to chlorine and other traditional drinking water disinfectants.
The disinfection of public water supplies still relies predominantly on chlorine, but also employs alternative disinfectants chlorine dioxide, monochloramines and ozone to treat water intended for human consumption. (White, G. C. 1999) Chlorine, chlorine dioxide and ozone are used at the treatment plant, sometimes sequentially, as “primary” disinfectants, to achieve water quality targets in the finished water—that is, at the point where the water leaves the plant. Chlorine and monochloramines are added to the water as “secondary” disinfectants, in order to maintain the quality of the distributed water all the way to the customer. In general, the anti-microbial efficacy of each of these disinfectants increases with temperature, approximately doubling with each 10° C. increase in water temperature. This finding is consistent with the Arrhenius equation, a well-known formula for the temperature dependence of reaction rates.
Chlorine is the chemical the most frequently used to disinfect public water supplies. The pH of the water being treated with chlorine greatly affects its disinfection efficacy. Chlorine dissolved in water exists as three species in equilibrium—chlorine gas (Cl2), hypochlorite ion (OCl−) and hypochlorous acid (HOCl). The ratio of the three components depends on the pH of the water. At pH below 2, chlorine gas becomes significant. When the pH is between 2-7, the equilibrium strongly favors hypochlorous acid, an effective antimicrobial agent. As pH increases above 7, hypochlorous acid dissociates to form hypochlorite ion, which has poor anti-microbial properties. At pH>8, hypochlorite ion dominates. Therefore, when chlorine is used to disinfect water, pH must be controlled to a lower pH in order to assure that hypochlorous acid, the anti-microbial species, predominates. The amount of chlorine that remains after the initial oxidant demand of the water is satisfied is known as the “free residual concentration”. EPA regulations allow a free residual chlorine concentration in potable water of up to 4 mg/L. Chlorine at allowable dose levels has proven effective for inactivating a broad range of traditional (fecal-borne) pathogens in drinking water, Cryptosporidium parvum, an encysted protozoan enteric parasite, is the notable exception.
Chlorine dioxide is a relatively powerful, fast-acting disinfectant, which inactivates pathogens across a broad pH range, from about pH 5 to 9. Chlorine dioxide sometimes is used as an alternative to chlorine for primary disinfection; however, the ability of chlorine dioxide to persist in the distribution system is unclear. Chlorine dioxide typically is not used in the United States for secondary disinfection; however, it has been used as a secondary disinfectant in several European countries including Italy, Germany, France, and Switzerland.
The amount of chlorine dioxide that remains after the initial oxidant demand of the water is satisfied is known as the “free residual concentration.” EPA regulations allow a free residual chlorine dioxide concentration in potable water of up to 0.8 mg/L. Chlorite ion, the EPA-regulated disinfection by-product of chlorine dioxide, has a maximum allowable concentration in potable water of 1.0 mg/L, which effectively limits the dose of chlorine dioxide that can be used to treat drinking water. The anti-microbial efficacy of chlorine dioxide at pH 5-9 for a broad range of traditional, fecal-borne pathogens in drinking water is roughly comparable or superior to that of chlorine at pH 5-7. Chlorine dioxide is more effective than chlorine against Cryptosporidium. Chlorine dioxide is highly soluble in water but, unlike chlorine, chlorine dioxide does not react with water (hydrolyze); rather, it exists as a dissolved gas. Chlorine dioxide at STP is approximately 10 times more soluble in water than is chlorine; the solubility of chlorine dioxide increases as the temperature of the water decreases.
Monochloramine is an oxidant sometimes used as a secondary disinfectant, in order to maintain a relatively weak but persistent disinfectant residual throughout a distribution system. Monochloramine reacts with organics at a much slower rate than chlorine; it is therefore is often part of a strategy for minimizing formation of regulated disinfection by-products associated with chlorine. The anti-microbial efficacy of monochloramines at for a broad range of traditional, fecal-borne pathogens in drinking water is far less than that of chlorine or chlorine dioxide. (Van der Wende and Characklis, 1990)
The relative efficacy of chlorine, chlorine dioxide and monochloramines against biofilms and biofilm-associated organisms is different than vs. traditional pathogens. Information on chlorine dioxide efficacy against biofilms is inconsistent, though generally seems to be superior to that of chlorine. Chlorine has limited ability to penetrate biofilms or to inactivate biofilm-resident bacteria, while monochloramine is reportedly able to penetrate and inactivate organisms within biofilms. M. avium, an NTM species, is more resistant to chlorine than indicator bacteria and survives in distribution systems despite ambient chlorine residual concentrations; most strains appear to be more resistant to monochloramine compared to free chlorine. All NTM species are believed to be at least 100-fold more resistant to chlorine and other disinfectants compared to E. coli (Taylor et al., 2000).
Microbial control treatments applied to plumbing systems fall into two general categories, (1) acute and (2) continuous. Acute treatments typically are short-term interventions designed to remediate bio-contamination; continuous treatments typically are part of routine operations, intended to control bio-contamination. For premise plumbing, conventional acute treatment options have been thermal and chemical. Acute treatment generally been limited to emergency decontamination of premise plumbing systems associated with disease outbreaks, owing to the attendant health and safety dangers and damage to the physical plant (e.g., severe corrosion). (White, G. C. 1999)
High-temperature water (e.g., 170° F./77° C.) is sometimes used for acute treatment of domestic hot water systems, in a procedure called “thermal shock” or “super heat and flush”. Thermal shock carries significant scalding hazards, is difficult to implement and can cause serious damage to plumbing systems. The high temperatures required to kill plumbing-associated pathogens, such as Legionella, are difficult to achieve and maintain for sufficient time consistently throughout all portions of a premise plumbing system. Even when target temperatures are achieved, thermal shock does not remove established biofilms.
Chemical disinfectants are sometimes used at higher-than-usual doses for acute treatment of pathogen-colonized potable water systems in a process called “chemical shock”. The most frequently practiced form of chemical shock is “hyper-chlorination” using chlorine. The relatively high concentrations of chlorine employed reportedly cause corrosion, create leaks and otherwise adversely affect plumbing materials. Potable water systems are likely to be re-colonized within several weeks after hyper-chlorination. (Williams et al., 2011) Even when pH and chlorine concentration targets are achieved, hyperchlorination is reportedly ineffective at removing established biofilms. In most hyperchlorination protocols, chorine is used at doses sufficient to develop a free chlorine residual of at least 5 mg/L (up to 50 mg/L or more) that is maintained for up to 24 hours. Because chlorine efficacy is pH dependent, the water must be maintained at less than pH 8 and preferably less than pH 7.2. Application of such high concentrations of chlorine is likely to corrode pipes and damage plumbing system components, especially at the preferred pH levels where hypochlorous acid predominates. When flushed through taps, chlorine at the levels used for hyperchlorination can off-gas significantly with release of chlorine fumes substantially above OSHA limits.
A study of acute treatment of a hospital premise plumbing system used a shock dose of 50-80 mg/mL chlorine dioxide applied over an 8-hour period under acidic (low pH) conditions; the protocol included flushing of all outlets at 50-80 mg/mL for approximately 1 hour. Biofilm reportedly was reduced significantly in the cold and hot taps, but not eliminated; treatment of the showerheads was reportedly unsuccessful, with >3000 cfu/ml recovered. (Walker et al., 1997) When flushed through taps, chlorine dioxide at the levels employed in the study can off-gas significantly with release of chlorine dioxide fumes substantially above OSHA limits.
Chlorine, chlorine dioxide and monochloramines are used for continuous treatment of potable water inside of buildings, especially of domestic hot water. Studies with continuous application of chlorine dioxide in a hospital potable water system showed that an extended time (>12 months) was needed to achieve significant reduction in Legionella positivity in hot water system. (Srinivasan, et al., 2003)
The net present replacement value of premise plumbing is in excess of $0.6 trillion (NRC, 2006). Moreover, costs associated with premise plumbing failures due to corrosion are unpredictable, and include costs of property damage and mold growth. Corrosion of copper pipe, an important plumbing material, is a function of a number of complex variables and not fully understood. Chlorine, however, is known to be corrosive to copper pipe. At low pH where hypochlorous acid predominates, chlorine corrosion can be severe.
Mixtures of chlorine and chlorine dioxide have been reported in the literature but only in the context of making possible the use of cost advantageous processes for the production of chlorine dioxide or for minimizing the formation of toxic disinfection by-products in bulk water. For example, Rosenblatt et al. discloses the production of a mixture of chlorine and chlorine dioxide using a relatively low cost sodium chlorate-based process, with subsequent conversion of the chlorine component to chloramines by the addition of ammonia, or alternatively separation of the chlorine and chlorine dioxide, in order to remove chlorine and thereby avoid undesirable downstream effects (such as malodors) associated with chlorine-contaminated chlorine dioxide in distribution systems. (Rosenblatt, et al., 1994) Rittman et al. provides for the use of mixtures of chlorine and chlorine dioxide to minimize the formation of regulated disinfection by-products associated with chlorine treatment of drinking water in a water treatment plant. (Rittmann et al., 2002) In neither case is the use of a mixture of chlorine and chlorine dioxide for biofilm treatment taught, nor is the application of the mixture to premise plumbing. Katz et. al applied an equal dose of chlorine dioxide and chlorine at pH<7.2, conditions under which hypochlorous acid predominates, to disinfect the effluent from a municipal sewage treatment plant. (Katz et al., 1994) The Katz et al., results showed that the combination produced relatively-stable residuals of both disinfectants, and reduced the concentration of an undesirable disinfection byproduct. Katz et. al does not, however, teach use of the mixture for biofilm treatment, nor the application of the mixture to premise plumbing. In a study of the inactivation of Legionella in a model plumbing system, a combination of chlorine and chlorine dioxide did not show significant synergistic effect. (Zhang, 2007) Norgaard describes the use of chlorine dioxide to treat biofilms but never suggests combining with chlorine. In fact, Norgaard states that biofilm is unaffected by chlorination and points out disadvantages of using chlorine to treat premise plumbing due to its corrosive properties at low pH. (Norgaard, 2012).
In order to avoid excessive system noise and the possibility of erosion-corrosion, the generally accepted limits for flow velocities of domestic water are 8 feet per second for cold water and 5 feet per second in hot water, up to approximately 140° F. In systems where water temperatures routinely exceed 140° F., lower flow velocities such as 2 to 3 feet per second should not be exceeded.